The possibility of using a laser beam as a lightning rod, or to "invite" lightning strikes from thunderclouds, was investigated by L. M. Ball in 1964. Lightning "invitation" by other methods had previously been considered by M. Brook, et al., "Artificial Invitation of Lightning Discharges, "Journal of Geophysical Research, Vol. 63, No. 11, Nov. 1961, pp. 3967-3968, which is hereby incorporated by reference. On the basis of anticipated advances in the state of the laser art as forseen from the viewpoint of that time, and the erroneous assumption that collisional ionization was required within the laser beam, it was erroneously concluded that the laser lightning rod was impractical. Correspondence from that time related to this early investigation is included in U.S. Patent Office Invention Disclosure Document No. 10,574, 4 May 1972, by L. M. Ball, submitted approximately 8 years after the early investigation under the recently established U.S. Patent Office Disclosure Document Program.
Unforseen advances in the state of the laser art, improved theoretical understanding of the necessary processes, and outcomes of certain key experiments all serve to reverse the former conclusion. The laser lightning rod must now be regarded as a physically realizable device. One possible embodiment of a laser lightning rod is mentioned, but not claimed, in U.S. Pat. No. 3,719,829, by Jack Roy Vaill, dated 6 Mar. 1973, which is hereby incorporated by reference. The Vaill patent is based upon experiments which have been described by Koopman & Wilkerson, "Channeling of an Ionizing Electrical Streamer by a Laser Beam", Journal of Applied Physics, Vol. 42, No. 5, April 1971, pp. 1883-1886, which is hereby incorporated by reference. These experiments were incorrectly performed, and their outcomes erroneously interpreted, leading to the false conclusion that excessively high concentrations of ionization would be required in the laser beams. The embodiment described, but not claimed, in the Vaill patent relates specifically to a scheme for combining the beams of lasers of two distinct types for the purpose of achieving such excessively high concentrations of ionization via collisional processes. The Vaill patent, as it relates to the present application for patent, is therefore extremely narrow in scope. A further deficiency of the embodiment mentioned in the Vaill patent is that the lasers used are not adequately protected from damage by the lightning strikes they might intercept.
Those unfamiliar with the language of lightning literature may find a few definitions to be helpful. A cumulonimbus cloud or thundercloud may contain one or more thunderstorm cells, which are regions containing both updrafts and downdrafts and which tend not to mix with adjacent cells. Such a cell may be a source of rain, hail, or lightning, and may have a lifetime of many minutes before dissipating, after which new cells usually develop nearby. Lightning strikes to the ground are preceeded by things called stepped leaders, which are faintly luminous fingers of ionization which propagate in a stepwise fashion from cloud to ground. The steps may range from a few to hundreds of meters in length, with pauses of several microseconds between steps. When the stepped leader reaches the ground, a return stroke (a highly luminous region of intense ionization) propagates upward from ground to cloud along the path previously followed by the stepped leader. A lightning channel is thereby established. Lightning channel currents can be as large as 500,000 Amperes, resulting in explosive heating and expansion of channel gases. This expansion is the origin of thunder. A distinction is customarily made between a stroke and a flash. A flash is what is perceived by eye and brain, and it may be composed of up to 26 separate strokes, or current surges as detected with high speed measuring devices, all in the same spark channel. Usually there are only three or four strokes per flash, and sometimes only one. If, for any reason, the time separation of strokes becomes large enough to allow the channel core temperature to fall below and about 2000.degree. K, (and the free electron concentration to fall below some corresponding value) a new channel may form (preceded by a new stepped leader) along a path different from the old. A lightning flash may be resolved into much more complex structure than we describe here, involving a leader of another type, but the essential fact is that the stepped leader always precedes and determines the path ultimately followed by the lightning channel. For further details see Martin. A. Uman's book Lightning, McGraw-Hill, 1969, which is hereby incorporated by reference